DIVERGENCE IN INSECTS. 225 



III. Divergence in Insects. 



The dependence of divergence on some form of isolation is most 

 clearly exemplified in insects, and though my studies are but limited in 

 that field, I shall refer to a few cases, which may serve to direct atten- 

 tion to a class of facts of the highest interest not only to entomology, 

 but to general biology. 



1. Divergence in Erynnis and Thanaos. 



Erynnis (Pamphila) and Thanaos (Nisoniades) . — These two genera 

 of small North American butterflies are worthy of the special atten- 

 tion of those who are studying the problems of divergent evolution, for 

 they furnish strong indications that organisms which are with diffi- 

 culty distinguished from each other by external form or color may, nev- 

 ertheless, be well-established species segregated presumably by sexual 

 instincts corresponding to sexual characters by which those of opposite 

 sexes of the same species readily recognize each other, and probably 

 cut off from the possibility of producing hybrids through incompati- 

 bility of physiological endowments. In the origin of some of these 

 species geographical isolation may have had an important influence ; 

 but concerning others there can hardly be a doubt that the segregative 

 influences, holding apart species that occupy the same districts, were, 

 from an early stage, peculiarities of their sexual instincts and constitu- 

 tion. The reason for accepting this view of their origin is found in the 

 fact that, though slightly divergent in other points, the characters by 

 which they are clearly distinguished are found in the forms of the male 

 genitaUa ; and in the characters of these organs we find clearly marked 

 species, for the most part free from the intergrading forms which would 

 certainly be presented if the different species were not prevented from 

 crossing by sexual instincts or constitution. 



A full description of these genera, with observations on the asymmet- 

 rical devolopment of the right and left sides of the genital armature 

 in Thanaos, will be found in Scudder's Butterflies of New England. 

 (See also Mem. Boston Soc. Nat. Hist., 11 (1874), and Proceedings of the 

 same Society for April 27, 1870, vol. xin (1871), p. 282. 



2. Divergence in Basilarchia. 



(i) Basilarchia (Scudder) is an attractive genus of butterflies pecu- 

 liar to North America, where it is represented by four or five species. 

 Three of these are found in New England, and are minutely described 

 in Scudder's "Butterflies of New England," from which I draw my 

 information (pp. 250-305).. 



